This invention relates generally to the field of gas turbine engines, and more particularly to the control of the combustion process within a gas turbine engine in order to avoid destructive dynamics and to minimize undesirable emissions.
The products of combustion of fossil fuels include carbon dioxide, carbon monoxide, unburnt hydrocarbons and nitrogen oxide (NOx), among others. Various control schemes and hardware configurations have been used to control the concentration of such emissions while at the same time providing fuel-efficient and stable engine operation. Regulatory changes continue to reduce the allowable level of emissions from electric power generating plants utilizing gas turbine engines. Gas turbine power plants in most locations must now be operated to produce no more than 15 ppm NOx, and in some locations, to produce no more than 7 ppm NOx or even 3.5 ppm NOx. Carbon monoxide emission limits can be as low as 10 ppm. To achieve such low levels of emissions, it is necessary to establish and to maintain very lean combustion conditions. Lean combustion is known to be less stable than rich combustion, and lean-burn combustors are more prone to damaging pressure pulsations generated within the combustor. Precise xe2x80x9ctuningxe2x80x9d of the combustion process is needed to establish a balance between stable combustion and low emissions. A precisely tuned engine may be susceptible to drift over time, with a resulting increase in emissions or an increase in the level of combustion instability.
One known approach to controlling the emissions from a gas turbine power plant is to install a catalyst downstream of the turbine. With this approach, the combustor can run at a relatively rich setting, thereby ensuring stable combustion while generating excessive amounts of undesirable emissions. The exhaust gas is then cleaned to regulatory limits by passing it through a combustion catalyst installed downstream of the combustor in the turbine exhaust system. Catalyst systems are expensive and are often used as a last resort in especially rigorous regulatory situations.
The generation of NOx emissions is directly related to the peak flame temperature in the combustor. For more than two decades it has been known to control the peak flame temperature in a gas turbine combustor by injecting water into the combustor. U.S. Pat. No. 4,160,362 dated Jul. 10, 1979, describes a gas turbine power plant having a reduced level of nitrogen oxide emissions. The gas turbine power plant described in that patent includes a system for controlling the amount of water injected into the combustor as a function of gas turbine load corrected for variations in compressor inlet temperature, i.e. ambient temperature. Temperature compensation is also known for determining the pilot fuel fraction in a dual-mode gas turbine combustor. The present inventors are unaware of any prior art gas turbine system that accommodates changes in the moisture content of the ambient air.
Further improvements in gas turbine engines and control schemes are desired to provide stable, reliable operation at ever-decreasing emission levels.
A method of controlling a dual-mode gas turbine combustor having a pre-mixed combustion zone and a pilot diffusion combustion zone is described herein. Pilot fuel fraction is defined as a ratio of a fuel flow rate to the pilot diffusion combustion zone divided by a sum of the fuel flow rate to the pilot diffusion combustion zone plus a fuel flow rate to the pre-mixed combustion zone. The method described herein includes controlling the pilot fuel fraction as a function of a level of humidity of ambient air used for combustion in the combustor. The method may further include controlling the pilot fuel fraction in response to a measurement of relative humidity of the ambient air, or in response to measurements of relative humidity and temperature of the ambient air. The method may include controlling the pilot fuel fraction as a function of specific humidity of the ambient air, or controlling the pilot fuel fraction in response to respective measurements of dry bulb temperature, relative humidity and barometric pressure of the ambient air. The method may further include controlling the pilot fuel fraction in response to respective measurements of dry bulb temperature, wet bulb temperature and barometric pressure of the ambient air.
A gas turbine engine having a dual-mode combustor is described herein as including: a pre-mixed combustion zone; a pilot diffusion combustion zone; a sensor for producing a humidity signal responsive to a level of humidity of ambient air used for combustion in the combustor; and a fuel system for providing a pilot fuel fraction responsive to the humidity signal, the pilot fuel fraction being a ratio of a fuel flow rate provided to the pilot diffusion combustion zone divided by a sum of the fuel flow rate provided to the pilot diffusion combustion zone plus a fuel flow rate provided to the pre-mixed combustion zone. The sensor for producing the humidity signal may be a relative humidity sensor. Alternatively, the sensor for producing a humidity signal may include: a dry bulb temperature sensor for producing a dry bulb temperature signal; a relative humidity sensor for producing a relative humidity signal; a barometric pressure sensor for producing a barometric pressure signal; and a calculator for generating the humidity signal responsive to the dry bulb temperature signal, the relative humidity signal and the barometric pressure signal. The sensor for producing a humidity signal may include: a dry bulb temperature sensor for producing a dry bulb temperature signal; a wet bulb temperature sensor for producing a wet bulb temperature signal; a barometric pressure sensor for producing a barometric pressure signal; and a calculator for generating the humidity signal responsive to the dry bulb temperature signal, the web bulb temperature signal and the barometric pressure signal.
A method of controlling a gas turbine engine is described herein as including controlling a fuel flow to a combustor of a gas turbine engine in response to a level of humidity of ambient air used for combustion in the combustor. The method may include controlling a ratio of fuel and air in at least a portion of the combustor in response to the level of humidity. The method may include controlling a pilot fuel fraction of a total fuel flow rate to the combustor in response to the level of humidity.